1. Technical Field
The invention is concerned with fabrication of silica-based optical fiber. It contemplates use of a composite preform including a core rod and a sol-gel-derived overcladding tube.
2. Description of the Prior Art
Optical fiber has become an extremely sophisticated product. Insertion loss has been lessened to increase the length of repeaterless spans. Dispersion has been addressed to result in significant increase in capacity. Strides have been made in fiber integrity. A number of developments in the glass itself, as well as in coating materials, have resulted in a high proof-test, bend-tolerant product. For the most part, advances have been accompanied by cost increase.
Significant cost saving is due to realization that the functioning portion of the fiber is small. Prevalent single-mode fiber structures owe their performance characteristics to but a few percent of the total cross-section of the fiber. One manufacturing approach depends on drawing the fiber from a composite preform made up of a core rod encompassed within a separately prepared overcladding tube. Since little of the transmitted energy sees the outer region of the clad, requirements on this region are reduced. A commercially available overcladding tube depends on a massive tubular starting body, drawn to small diameter tubing, which, together with the core rod, makes up the composite preform.
Further cost saving will likely result by use of a sol-gel-derived overcladding tube. U.S. Pat. No. 5,240,488 issued Aug. 31, 1993, represents the breakthrough to finally realize promised cost saving. The primary obstacle to earlier use--reduced yield due to cracking of any but the very smallest bodies--is avoided by incorporation of a transient polymer in the sol. Having served to prevent cracking during drying, the polymer is removed during firing.
Fiber breakage, due to very small numbers of very small particles--contaminant particles, now carried over to the glass (in the absence of the vapor transport step of previous fabrication methods)--is avoided by particle removal, by physical separation and/or by gas reaction. (See U.S. patent applications Ser. Nos. 08/029750 and 07/029753, both filed Mar. 11, 1993.)
A final problem has arisen. The near-net shape, cast overcladding tube is subject to warpage which interferes with insertion of the closely titling core. Uneven cooling and minor differences in temperature coefficient of expansion as due to small deposits of SiO.sub.2 of differing morphology--e.g., of cristobalite--are among the causes.